Alkene ozonolysis SOA: inferences of composition and droplet growth kinetics from Köhler theory analysis

Abstract. The CCN properties, surfactant characteristics, and droplet growth kinetics of secondary organic aerosol (SOA) formed from the ozonolysis of three parent alkene hydrocarbons (terpinolene, 1-methlycycloheptene and cycloheptene) are explored. Based on measurements of CCN activity, total carbon and inorganic ion concentrations, we estimate the average molar volume of the water-soluble organic component using Köhler Theory Analysis (KTA). The results suggest that the water-soluble organics in the SOA are composed of relatively low molecular weight species, with an effective molar mass less than 200 g mol−1. This finding is consistent with the speciated fraction for some of the SOA, and suggests that KTA can be applied to complex organic aerosol, such as that found in the atmosphere. From measurements of CCN activity and Köhler Theory, we apply a novel method to infer the surface tension at the point of activation; this is used to infer the presence of surface-active organics. It is found that the water-soluble carbon can be surface-active, depressing surface tension 10–15% from that of pure water at concentrations relevant for CCN activation. Although important, this level of surface tension depression is lower than expected for HULIS, which suggest that they are not likely in the SOA examined. In all cases, the CCN exhibit droplet growth kinetics similar to (NH4)2SO4.